Among other aspects, a laser cutting process depends on the quality and temporal behaviour of a focussed laser beam used in the laser cutting process. Lenses and/or mirrors are used to guide and form the laser beam. Thus, the lenses and/or mirrors influence the resulting cut. Wear on the lenses (or on other optical elements in the beam path) can increase the absorption of the laser beam by the lenses or other optical elements, which results in heating of the lenses or other optical elements. The heating can cause changes in the optical properties of the lenses or other optical elements, such as, for example, a greater focal shift.
Because a change in the temperature of the optical element frequently causes a change in the condition of the optical element, various techniques have been proposed to detect the temperature of an optical element or a parameter describing this. For example, a resistance measurement is disclosed in European Patent No. EP 1 310 782 A1. A change in a physical parameter of an optical element such as, for example, the temperature, due to the absorbed radiation is sensed and used as a source of information of the condition of the optical element. In this method the temperature is determined by measuring an electrical resistance. In German Utility Patent No. 203 14 918 U1, a device for monitoring a protective glass in a system of laser optics for breakage and/or contamination is disclosed. In German Patent Application No. 10 2004 006 565 A1, a change in length of an optical element is measured. A temperature change that is due to a change in the optical properties is determined based on the measured change in length.
In German Patent Application No. 197 094 73 A1, a degree of wear of a lens arrangement is determined during the machining of a workpiece with a laser beam. The workpiece is cut with the laser beam, and a cut-in time required to cut through the workpiece is measured and compared with a predetermined reference cut-in time to generate, for example, a warning signal if both cut-in times deviate from each other by more than a given tolerance value. A trace on the workpiece is generated with the laser beam, and at different times the radiation coming from the workpiece is measured, in order to deduce the wear condition of the lens arrangement by comparing the measured radiation values taken at different times. In International Patent Application Publication No. WO 98/33059, a photodetector detects scattered light generated by contamination.